In general, a process of frothing milk involves mixing the milk with air, so that air bubbles are formed in the milk, and a foamy milk substance is obtained. In view of this fact, a device for frothing milk needs to be adapted to take in air and supply the air to a quantity of milk. In many cases, for the purpose of taking in air, the milk frothing device has an air restriction, which may be a suitable orifice, for example. However, one of the main difficulties in milk frothing devices is ensuring that the air restriction is reliable, wherein the air restriction should not vary in time, and should not suffer from cleaning issues.
According to one possibility, the air restriction may be arranged such as to be separated from the environment of the milk frothing device, and to be connected to a frothing geometry by suitable transport means. In that case, a difficulty resides in the fact that milk has to be removed from the transport means once it enters there, which may happen under the influence of pressures prevailing in the frothing geometry, for example.
According to another possibility, the air restriction is located in the direct vicinity of a frothing geometry, wherein there is no need for transport means. In that case, the air restriction may be embodied as a combination of a pin and a hole, wherein the pin has a flat side, and wherein the pin is inserted in the hole. For cleaning purposes, the pin may be removed from the hole. Hence, cleaning is a rather easy task, since no small holes or channels that are difficult to reach are involved.
U.S. Pat. No. 4,715,274 discloses an emulsifier unit for application to coffee making machines. The emulsifier unit comprises a suction chamber whereto a milk intake duct and an air intake duct are led. In order to control a working cross-sectional area for the air flow, the emulsifier unit further comprises a pin having a facet formation insertable into the air intake conduit. U.S. Pat. No. 4,715,274 teaches that with this arrangement, it becomes possible to provide the air intake conduit with a relatively large cross-sectional area, whereby this conduit can be easily and quickly cleaned to prevent stagnation of any fat particles or the like. Furthermore, U.S. Pat. No. 4,715,274 teaches that the pin may be used to change the working cross-sectional flow area by merely turning it.
Still, a problem of the use of the pin-and-hole air restriction in a milk frothing device is that blockage may occur during operation by a gelation process of milk that is drawn in by capillary forces. The pin is in constant contact with milk, and once the milk dries in, it starts to form a milk-gel, whereby the restriction may be completely blocked.